Pascal’s Principle“Pascal’s Principle states that if the pressure at any point in a liquidthat is enclosed and at rest, is changed, then the pressure at allpoints in the liquid is changes by the same amount” (OTEN, 2002c, p.13).“Pressure is described mathematically as a force exerted over an area(or P = F ÷ A) and is measured I N/mm2 or Megapascals (MPa)”(Metcalfe & Metcalfe, 2009, p. 172).
Pascal’s Principle“According to Pascal’s Principle, a force applied to a confined fluid istransmitted in all directions throughout the fluid regardless of theshape of the container. Effectively this is shown in the systemdepicted in the image below. If the input piston is forceddownwards, a pressure is created throughput the fluid, which actsequally at right angles to the surface in all parts of the system”(Metcalfe & Metcalfe, 2009, p. 172).
Pascal’s Principle“If two cylinders connected (as shown in Figure 3.12) have a forceapplied to the smaller cylinder, this would result in a given pressure.By Pascal’s Principle, this pressure would be identical to the pressurein the larger cylinder. Because the larger cylinder has more area, theforce emitted by the second cylinder would therefore be greater”(Metcalfe & Metcalfe, 2009, p. 172).
Pascal’s PrincipleThis is represented by rearranging the pressure formula: P=F÷A to F = PABecause the pressure stayed the same in the second cylinder (butarea was increased) a larger resultant force was generated, includinga mechanical advantage” (Metcalfe & Metcalfe, 2009, p. 172).
Archimedes’ PrincipleArchimedes’ Principle states that “when a body is wholly or partiallyimmersed in a fluid, it is acted upon by an upthrust which is equal tothe weight of the fluid displaced. This upthrust, or buoyancy, actsthrough the centre of mass of the displaced fluid. The centre of massis therefore referred to as the centre of buoyancy” (OTEN, 2002c, p.16).
Additional Reference Material• Copeland, P. L. (2002). Engineering studies: the definitive guide. (Vol. 1: the preliminary course). Helensburgh NSW: Anno Domini 2000 Pty Ltd.• Metcalfe, P., & Metcalfe, R. (2009). Excel senior high school engineering studies. Glebe NSW: Pascal Press.• OTEN. (2002c). Part 4 : engineering mechanics, hydraulics and communication - 2. Braking systems. Retrieved 12 May 2012, from https://portalsrvs.det.nsw.edu.au/f5-w- 687474703a2f2f6c72722e646c722e6465742e6e73772e6564752e 6175$$/LRRDownloads/2938/1/41082ES_MOD%203%20BS%20Pa rt%2004.doc
Examples of the usesof hydraulic principlesin braking systems
Examples of the uses of hydraulicprinciples in braking systems“Pascal’s Principle is particularly important in automobile brakingsystems. Most automobile braking systems are closed hydraulic systemsworking on Pascal’s Principle” (Copeland, 2002, p. 90).“If we consider the car breaking system as a closed hydraulic systemthen we can apply Pascal’s Principle. The brake pedal is connected to themaster cylinder assembly under the bonnet. This consists of a piston in acylinder. By pressing the pedal, the piston is forced into the cylinder soincreasing the pressure in the master cylinder. This forcing the padsagainst the discs and so applying the brakes” (Copeland, 2002, p. 90).
Disc BrakesThe disc brake is the most common type ofbrake used in modern personal and publictransport vehicles such as cars and buses.Disc brakes consist of “a rotating disc (rotor)that is connected to the axle. Connected to the suspension is a backing plate with a caliper attached. The caliperwraps over the disc and houses two pads that are forced laterally againstthe disc by a hydraulically operated piston. The frictional resistancecreated retards the rotor. The disc brake offers better heat distributionthan the drum brake and also offers better wet-weather performance aswater is thrown off the disc by centrifugal force. Initially, the disc (rotor)was solid but now they have vents through them (ventilated discs) orthey are drilled to further improve heat distribution”(Copeland, 2002, pp. 78-79).
Disc Brakes“One disadvantage of disc brakesis that they have no naturalservo-assistance, so the force atthe pedal is very large. To reducethe effort the pedal force is“boosted”. A vacuumbooster, running off the enginemanifold, achieves this. Thismagnifies the pedal effort thedriver providesand improves stopping performance. Should a vacuum boosterfail, then the effort needed to stop a disc brake car is very large. Discbreaks are also not good as hand brakes du to the lack of servo-assistance” (Copeland, 2002, pp. 78-79).
Anti-lock Braking Systems (ABS)One of the biggest innovations inbraking systems was the successfuldevelopment of Anti-lock brakingsystems (ABS). These brakingsystems prevent the tendency of acar to skid under braking. “When acar skids in dry conditions it stopsquickly but not necessarily in a straight line. The unequal braking forcesat the wheels cause a turning moment about the car’s centre ofrotation, which tends to cause the car to turn “off-line” or swerve. In thewet the wheels lock far more quickly and at a point way below maximumbraking force. With a front wheel lock-up the car loses the ability to steerwhile the rear wheel lock-up often causes the tail to swing out. Bothsituations can lead to a loss of control and consequently accidents”(Copeland, 2002, p. 79).
Anti-lock Braking Systems (ABS)“Car makers and brake designers have attempted to design brakingsystems that will not lock up. This is usually achieved by using wheelsensors and computer control over the braking circuit. When a wheelbegins to lock up and slow down, unlike other wheels, a wheel sensormounted on the wheel will sense this. It will send a message to thecomputer to inform it of a wheel lock-up. The computer will thenrelease the pressure at the brake caliper to allow the wheel to spinagain. The brakes can then be reapplied and if the wheel locks againthe procedure is repeated” (Copeland, 2002, p. 79).
Anti-lock Braking Systems (ABS)In Australia there is some concernabout the performance of ABS brakeson gravel roads. Gravel roads needthe wheels to lock and dig throughthe loose surface to the hardersurface beneath, but as an ABS brakeequipped car will not allow this tohappen, an ABS equipped car will notstop as well on gravel. This is usually not a major concern for Japaneseor European car designers. Many cars with ABS also use the samesensor to control wheel spin under acceleration” (Copeland, 2002, p.79).
CASE STUDY 2:FLUID MECHANICS IN LIFTING DEVICES
ElevatorsModern elevator or lift systems are driven byhydraulic or electric motion. “Hydraulic liftsare used extensively in low-rise buildings upto five stories, for example small apartmentblocks, clubs, nursing homes and hospitals.Speeds rarely exceed 0.75 m/s and nooverhead lifting gear is needed. They aresuitable for non-intensive duty designs”(OTEN, 2002b, p. 15).“Generally the total installation costs of hydraulic systems, includingbuilding costs, are less than for electric traction alternatives althoughperformance efficiency is generally not equal” (OTEN, 2002b, p. 16).
ElevatorsThe use of hydraulics in elevator systems has allowed people andmaterials to effortlessly more from floor to floor of a multi-storybuilding. This has been particularly useful for items which areunsuitable to be take up staircases due to their size, shape and/orweight.“Without the availability of an efficient elevator system the focus ofour construction methods would possibly change from constructingtall buildings in areas where land is scarce and expensive to a systemof low rise buildings constructed on less expensive land. This wouldhave the effect of increasing the urban sprawl as cities spreadoutwards in search of suitable building sites” (OTEN, 2002a, p. 29).
Elevators Direct acting hydraulic system Side acting hydraulic system
Cranes“Cranes are one of the bigger lifting devicesyou may see especially around largeconstruction sites. Scaled down cranes aremounted on the back of tow trucks to liftone end of a vehicle so it can be towed awayafter an accident. However, most cranes areused to lift things high off the ground, suchas for lifting materials to the upper floors ofa building under construction. All cranes aredesigned to lift a suspended load from oneplace to another” (OTEN, 2002a, p. 5).
CranesTelescopic extension cranes were a majorinnovation in crane design from the 1960s.Telescopic cranes have the advantage ofbeing able to work in confined spaces withthe boom extending only as far as needed.An early example from 1966 is seen in thepicture on the right.Telescopic cranes rely largely on the advantages of hydraulics for theireffectiveness. The hydraulic systems or theses cranes have “highefficiency ratings because there are few moving parts and friction isreduced by using oil-based fluids. Modern telescopic cranes canreach to heights of about 60 metres or further if a trussed jib isattached to the final boom extension” (OTEN, 2002a, p. 12).
Tower CranesAn innovation in lifting devices is the self-erecting tower crane. “Tower cranes have veryhigh towers or masts reaching unsupportedheights of 80 metres. Greater heights can be achieved ifthe mast is tied to the frame of the building at regularintervals” (OTEN, 2002a, p. 13).Tower cranes have a climbing frame just below the cabinthat uses “large hydraulic rams to lift the cabin and jib onemast section higher. The new mast section is lifted by thecrane itself into the position opened up by the climbingframe. Once the new section is bolted to the lowerportion of the mast the whole operation can continueupwards. When the crane is no longer required it simplyreverses the procedure to dismantle itself”(OTEN, 2002a, p. 13).
Tower CranesTower cranes do not have a highcapacity with 20 tonnes being aboutthe maximum lift rating. Their mainfunction is to move building materialsaround the construction site especiallyto the upper floors of tall buildings.Some tower cranes have horizontaljibs, which may reach 75 metres or more. This enables then to reachfrom one side of a building site to the other even though the crane baseremains stationary. When working at the extreme end of the jib thelifting capacity is reduced by at least half due to the greater turningeffect placed on the crane by the load” (OTEN, 2002a, pp. 13-14).Tower cranes have allowed engineers to cope with the increased heightsof modern buildings and reduce their construction times. These cranesalso help to maintain safe working procedures by relieving workers fromcarrying heavy loads.
Use ofhydraulics/pneumaticsas part of aeronauticalapplication
Wing Flap HydraulicsWing flaps and other components ofan aircraft are connected by complex and sophisticated hydraulic networks.“These networks contain high pressurefluids capable of withstanding below-freezing temperatures and are aidedby servos (small electrically operatedmotors) and fluid pressure sensors.The system provides pilots with power-assisted controls while thesensors which are normally connected to onboard computersystems, provide the pilot with immediate monitoring and feedbackon the position and status of these controls. Some of these controlsare designed with redundancy in mind. This means that if onehydraulic network fails in a key area, a second network can continuein its place.
Wing Flap HydraulicsAircraft hydraulic systems are simply a method of transmitting energyor power from one place in the aircraft to another. The systemsinvolve an arrangement whereby liquid under pressure is used totransmit this energy. Hydraulic systems take engine power andconvert it to hydraulic power by means of a hydraulic pump. Thispower can be distributed throughout the airplane by means of tubingthat runs throughout the aircraft. Hydraulic power can be used in anumber of ways and in a variety of situations” (Metcalfe & Metcalfe,2009, p. 235).
Wing Flap HydraulicsThe controlled movement of aircraft wing flaps is a good example ofthe use of hydraulic power. In this instance “a hydraulic pump willconvert engine power before an actuating cylinder converts thehydraulic power to mechanical power” (Metcalfe & Metcalfe, 2009, p.235) thus allowing the movement of the flaps in the requireddirections. Wing flap hydraulics
Wing Flap Hydraulics“An important part in the hydraulic system is the actuating cylinder. It’smain function is to convert hydraulic or fluid power into movement ormechanical power. Inside the actuating cylinder is a piston whosemotion is regulated by oil under pressure. The oil is in contact with bothsides of the piston head but at different and variable pressures.Through the use of a selectorvalve, high-pressure oil may bedirected into either side of the pistonhead. Which side of the piston headthis pressure is directed to willdetermine whether the shaft exerts apushing or a pulling force.Piston flaps may be connected from the actuating cylinder to wing flapsthus allowing the controlled movement of the flaps” (Metcalfe &Metcalfe, 2009, pp. 235-236).
Copeland, P. L. (2002). Engineering studies: the definitive guide. (Vol. 1: the preliminary course). Helensburgh NSW: Anno Domini 2000 Pty Ltd.Metcalfe, P., & Metcalfe, R. (2009). Excel senior high school engineering studies. Glebe NSW: Pascal Press.OTEN. (2002a). Part 1: lifting devices - developments. Lifting devices. Retrieved 13 May 2012, from https://portalsrvs.det.nsw.edu.au/f5-w- 687474703a2f2f6c72722e646c722e6465742e6e73772e6564752e6175$$ /LRRDownloads/3060/1/41094_ES_MOD_LD%2001.docOTEN. (2002b). Part 2: lifting devices - mechanics/hydraulics. Lifting devices. Retrieved 13 May 2012, from https://portalsrvs.det.nsw.edu.au/f5-w- 687474703a2f2f6c72722e646c722e6465742e6e73772e6564752e6175$$ /LRRDownloads/3061/1/41094_ES_MOD_LD%2002.docOTEN. (2002c). Part 4 : engineering mechanics, hydraulics and communication - 2. Braking systems. Retrieved 12 May 2012, from https://portalsrvs.det.nsw.edu.au/f5-w- 687474703a2f2f6c72722e646c722e6465742e6e73772e6564752e6175$$ /LRRDownloads/2938/1/41082ES_MOD%203%20BS%20Part%2004.doc